Atmospheric aerosols have significant effects on human
health and the climate. A large fraction of these aerosols originates from
secondary new particle formation (NPF), where atmospheric vapors form small
particles that subsequently grow into larger sizes. In this study, we
characterize NPF events observed at a rural background site of Hada Al Sham
(21.802
The effect of atmospheric aerosols on the Earth's radiative balance, via scattering, absorption and cloud interactions, is the single largest factor limiting our understanding of future and past climate changes (Stocker et al., 2013). In addition to the climate effects, aerosols are known to be detrimental to human health, with outdoor particulate pollution being the cause of more than 3 million premature deaths in the year 2010 (Lelieveld et al., 2015). These effects include the contribution of both primary and secondary aerosol particles. Primary particles are emitted into the atmosphere directly as particles, while secondary particles are formed from atmospheric vapors in new particle formation (NPF) events. Measurements of submicron particle number size distributions (PNSDs) have shown that NPF events are a global phenomenon (Kulmala et al., 2004), and they are estimated to produce around half (31 %–66 %) of the cloud condensation nuclei (CCN) in the lower troposphere (Yu and Luo, 2009; Merikanto et al., 2009; Gordon et al., 2017). Even in polluted regions, where the primary emissions are high, NPF events are found to occur frequently (Yu et al., 2017), and they are estimated to be a significant contributor to the particle number concentrations (Yue et al., 2011; Kulmala et al., 2016; Yao et al., 2018). Despite the importance of NPF, many aspects related to the initial formation and subsequent growth of secondary aerosol particles remain unknown. While sulfuric acid is widely regarded as the most important precursor for NPF, it is clear that other compounds are needed to explain particle formation and growth rates in ambient measurements, especially in the boundary layer (Kirkby et al., 2011; Ehrhart et al., 2016). Stabilizing bases, such as ammonia and dimethylamine, low-volatility oxidation products of VOCs (volatile organic compounds) and ions have been shown to enhance particle formation rates and bridge some of the gaps between theoretical evaluations, laboratory studies and ambient measurements (H. Yu et al., 2012; F. Yu et al., 2018; Almeida et al., 2013; Kürten et al., 2014, 2016, 2018; Zhang et al., 2004; Riccobono et al., 2014). The initial particle-forming compounds will also participate in growing the particles, but, in order to explain the observed growth rates, the presence of more abundant condensing (or otherwise particle mass forming, for example, via heterogeneous oligomer formation) species is required (Nieminen et al., 2010; Riccobono et al., 2012). These species are very likely to be low-volatility or semivolatile organic compounds, formed by the oxidation of either biogenic or anthropogenic VOCs (Smith et al., 2008; Tröstl et al., 2016; Dall'Osto et al., 2018). Overall, the mixture of compounds and the relative importance of different species participating in aerosol formation and growth are expected to vary depending on the ambient conditions; in some coastal environments NPF can be driven by iodine compounds (Sipilä et al., 2016), and, for example, in urban areas the uptake of nitrate can contribute significantly to aerosol mass (Li et al., 2018). Predicting all the occurring interactions in the atmosphere is impossible without observations from several different environments. While PNSD measurements have been conducted in a wide range of environments (Kerminen et al., 2018), especially continuous long-term measurements are still fairly uncommon and largely focused on Europe and the midlatitudes (Nieminen et al., 2018). Long-term measurements are needed for obtaining reliable estimates on the average properties and seasonal tendencies of atmospheric NPF. Such data are essential in evaluating the performance of global models, which currently have large uncertainties in simulating atmospheric NPF, as well as its contribution to CCN budgets and aerosol radiative effects in different environments (Pierce and Adams, 2009; Makkonen et al., 2012; Gordon et al., 2016; Semeniuk and Dastoor, 2018).
Recently, several NPF studies have pointed out an interesting phenomenon, where the average diameter of the particle mode formed in an NPF event begins to decrease after the growth phase. This is often referred to as aerosol shrinkage, but we will use the term DMD (decreasing mode diameter) event, since aerosol shrinkage quite directly implies a reduction in the size of individual particles, which is not necessarily the case. Such DMD events have been observed especially in subtropical regions (Yao et al., 2010; Backman et al., 2012; Cusack et al., 2013; Young et al., 2013; Zhang et al., 2016; Alonso-Blanco et al., 2017) but also in the temperate climate (Skrabalova, 2015; Salma et al., 2016). Typically, these DMD events are suggested to be caused by the evaporation of semivolatile compounds, due to changes in environmental conditions. However, the reduction in the mean diameter of a particle mode may also occur without evaporation, if smaller particles are transported to the measurement site (Kivekäs et al., 2016).
In this paper, we study the aerosol particle number size distribution dynamics at Hada Al Sham, Saudi Arabia, during February 2013–February 2015. The environment is quite unique due to high anthropogenic and low biogenic emissions, as well as a distinct segregation between the surroundings in different directions. To our knowledge, these are the first comprehensive long-term aerosol measurements conducted in the Arabian Peninsula. Two articles from the same measurement campaign are already published, describing the aerosol physical (Lihavainen et al., 2016) and optical properties (Lihavainen et al., 2017). The former article also contains analysis of particle number concentrations, which will not be presented in this study. This work focuses on identifying and characterizing NPF in the study region. We show that the NPF events in Hada al Sham are, in comparison to the locations analyzed in the exhaustive study by Nieminen et al. (2018), exceptionally frequent and intense, in terms of both particle formation and growth rates. We also make a detailed investigation of the typical diurnal cycle related to the NPF events and determine how the events are impacted by different environmental variables, including meteorological conditions and the concentrations and sources of primary aerosol particles and aerosol precursor compounds.
Hada Al Sham (21.802
The measurements were conducted at the Agricultural Research Station of King
Abdulaziz University from February 2013 to February 2015. The instruments
were placed inside a container, located on a sand field, with a distance of
NPF event classification was done for the measurement days based on the
visual interpretation of PNSDs, as described by Dal Maso et al. (2005). Each day was classified as either (1) an NPF day, (2) a nonevent
day or (3) undefined. In short, a day is classified as an NPF day if a new
growing mode of particles appears in the nucleation mode (
As an addition to the traditional classification, each NPF day was further classified based on whether the mean diameter of the mode formed in the NPF event clearly starts to decrease after the growth phase (see Fig. 1) or not. These days are referred to as DMD days and non-DMD days, respectively.
Particle number size distribution measured by DMPS showing an NPF
event with a decreasing mode mean diameter (after 15:00 LT; UTC
To describe the progression of the NPF events, we determined the points in time when (1) NPF is first observed in the smallest size bins of the DMPS measurements, (2) NPF is no longer observed, (3) the mode diameter of the newly formed particles starts to decrease and (4) the mode formed in the NPF event is no longer distinguishable from the background aerosols due to decreased number concentrations or, for example, changes in air masses. These times were determined visually from the PNSDs, and they are referred to as NPF start, NPF end, DMD start and NPF event end, respectively (Fig. 1). NPF start times were only determined for the days when NPF was observed all the way from the smallest size bins, and the NPF event end times were only determined for the days when the event ended during the same day as NPF had started.
The formation and growth rates of newly formed particles are important quantities in describing NPF (Kulmala et al., 2012). They provide information on the strength of the NPF events and are closely connected to the atmospheric factors driving the process, such as the concentrations of condensable vapors. The growth rates of freshly formed particles have a critical role in their probability to survive into climate-relevant sizes, as particles that grow too slowly are removed by coagulation with larger preexisting particles (Kuang et al., 2009).
In this study, the particle growth rates were determined by following the
time development of the geometric mean diameters (GMDs) obtained from
lognormal fits to the PNSD at each measurement time. The fitting was done
using an automatic algorithm developed by Hussein et al. (2005), which analyses the measured PNSD, fits 2–3 lognormal modes and
returns the fitting parameters. In practice, the growth rates are determined
by plotting the GMDs of fitted modes together with the PNSD and making a
linear fit to the points selected to represent the mode formed in the NPF
event (Fig. 1). Now, the diameter growth rate (GR) in the size range
The formation rate (
The coagulation sink describes the rate at which particles are lost due to
collision and coalescence with larger particles. The collisions can occur
due to differing settling velocities, turbulence, electric interactions or
Brownian motion. However, when describing the coagulation of submicron
particles in typical atmospheric conditions, the coagulation due to Brownian
motion is by far the most significant. When only this mechanism is
considered, the coagulation sink can be calculated by integrating over the
PNSD (Kulmala et al., 2001):
The condensation sink (CS) describes the ability of the aerosol population to
remove condensable vapors from the atmosphere. The concept is analogous to
the coagulation sink, defined with Eq. (3), but now, instead of the particle
loss rate, the rate at which vapors condense onto preexisting aerosol
particles is considered. Similarly to the coagulation sink, the CS is
calculated by integrating over the PNSD (Kulmala et al.,
2001):
Air mass history was studied by calculating particle retroplumes using a
Lagrangian particle dispersion model FLEXPART (FLEXible PARTicle dispersion
model) version 9.02 (Stohl et al., 2005). ECMWF (European
Centre for Medium-Range Weather Forecast) operational forecast with
0.15
The model was run for a time period from February 2013 to May 2014. During
this period, a new release of 50 000 model particles, distributed evenly
between 0 and 100 m above the measurement site, occurred every 1 h. The
released particles were traced backwards in time for 72 h, unless they
exceeded the model grid (0–45
The model output was saved every 1 h and, in relevance to this study, it contains (1) the emission sensitivity field, i.e., a matrix whose values are proportional to the time the model particles have spent over each grid point during the last hour; and (2) a point of the average trajectory that is determined by cluster analysis (Seibert and Frank, 2004) from the locations of the model particles. In addition, we use the atmospheric boundary layer (ABL) height at Hada Al Sham, obtained from the ECMWF operational forecast.
The results of the NPF event classification are presented in Fig. 2. The total NPF event frequency was found to be very high, as 73 % of all the classified days (454) were NPF days. Out of the NPF days, 76 % were DMD days, meaning that only about one quarter of the NPF events showed monotonic growth, which is the typical progression of NPF in most environments. Only 4 % of the days were classified as nonevents, leaving 23 % as undefined. It should be noted that the majority of the undefined days were likely days when new particles were formed, but continuous growth of particles was not observed at Hada Al Sham due to unfavorable meteorological conditions (referred to as “failed events” by Buenrostro Mazon et al., 2009).
Results of the NPF event classification as fractions of classified days, separately for each month and all of the classified days. The numbers above the bars show the number of classified days. Some of the months contain days from more than a year, while in some months (June, July, September and December) data were available only for a single year.
The monthly fractions in Fig. 2 show that the NPF frequency is high (55 %–85 %) throughout the year and that no clear seasonal pattern is observed. This implies that the NPF events are not limited by any factor with a strong seasonal variability. The most notable deviations from the average frequency are found in June, November and December, which all have a higher-than-average fraction of nonevent days. Although no seasonal cycle is seen in the total NPF frequency, the DMD events are more frequent during the summer (and autumn) months and less frequent during winter. The fraction of DMD events from all NPF days is highly variable, ranging from 33 % in November to 95 % in September.
The average NPF event frequency of 73 % in Hada Al Sham is among the
highest event frequencies obtained from long-term measurements. Nieminen
et al. (2018) compared PNSD measurements, consisting of at least 1 full
year of data, conducted at 36 different sites around the world. They
observed that NPF events are most frequent in South Africa, where the NPF
frequencies from three different sites were 69 %, 75 % and 86 % (Hirsikko
et al., 2012; Vakkari et al., 2011, 2015). Thus, the NPF
fraction of 73 % obtained from the measurements presented here would
take the third place in this global comparison. The high NPF event frequency
is a direct indication of typically favorable conditions for new particle
formation and growth. NPF event frequency has been shown to be affected by
at least solar radiation,
The connection between solar radiation and NPF events is related to
atmospheric photochemistry: the production of sulfuric acid, which is
regarded as the driving compound of atmospheric new particle formation in
most environments (Weber et al., 1997; Birmili et al., 2003; Kuang et al.,
2008; Paasonen et al., 2010; Yao et al., 2018), occurs mainly via oxidation of
Solar radiation alone is, of course, not sufficient to cause NPF if no
precursor vapors for the production of nucleating and condensing compounds
are available.
Average concentration of
In Fig. 4, we compare the air mass history during the mornings of NPF event
and nonevent days. The shown emission sensitivities are calculated from the
24 h retroplumes, initiated at the time when NPF is typically taking
place (10:00 LT). The comparison shows significant differences in the air
mass origins between these cases. On NPF days (Fig. 4a), the air masses
observed in Hada Al Sham originate mainly from a narrow strip that extends
along the coast and includes the regions of significant
Average 24 h emission sensitivity for air masses arriving at
Hada Al Sham at 10:00 LT for
The markedly different wind conditions between the NPF and nonevent days can also be seen from the local measurements at Hada Al Sham (Fig. 5a and b). On NPF days, the weak nocturnal easterly wind (land breeze) turns westerly and its speed starts to increase between 08:00 and 10:00 due to the development of the sea breeze. This shifts the air mass source regions to the coastal areas, as seen in Fig. 4a. On nonevent days, such change in the wind direction is not seen. During the night and early morning, the wind is easterly, similar to the NPF days, but, on nonevent days, the easterly wind is significantly stronger. This inhibits the development of the sea breeze circulation and the westerly wind associated with it.
Diurnal variation of
The strong easterly winds on nonevent days seem to resuspend dust from the
inland desert, which can be seen as the simultaneous increase in PM
Higher CS values during NPF days (Fig. 5c) are also reported from a high-altitude site in the Swiss Alps (Boulon et al., 2010). Here this connection is speculated to stem from the coupled appearance of NPF precursor vapors and CS due to their common lower-altitude sources. Similar situation could apply to Hada Al Sham if both the CS and the NPF precursor vapors originate mainly from the same sources. The high CS during the calm nights and early mornings of NPF days could then suggest enhanced accumulation of precursor vapors, thus facilitating the occurrence of the NPF process after sunrise.
Figure 6 displays the frequency histograms of the points in time describing
the progression of the NPF events (see Sect. 2.2.1), together with the
diurnal variation of meteorological parameters and CS. In Fig. 7, the NPF
progression times are plotted for a year-long period from June 2013 to June 2014 to show their seasonal variation. NPF events are typically observed to
start slightly before 09:00 (Fig. 6a). On a seasonal scale, the starting
times change according to the changes in the time of sunrise (Fig. 7). This
observation highlights the importance of photochemistry for the NPF,
especially since none of the NPF events start before sunrise. There is,
however, quite a significant day-to-day variation in the starting times, which
is not explained by differences in the times of sunrise. Some of this
variation can be attributed to differing growth rates between the NPF
events. This is because the NPF starting times are defined here as the times
when new particles are observed at the size of 7 nm, even though the
formation of new particles actually starts from the molecular scale.
Therefore, the time that it takes for the small particles/molecular clusters
of the size
The formation of new particles lasts, on average, for about 3 h ending around noon. The NPF end times are possibly affected by the simultaneously increasing CS (Fig. 6a and b), which is caused by the growth of both the freshly formed and the preexisting accumulation mode particles into larger sizes. With increasing CS, NPF precursor vapors and new small particles are more likely to end up contributing to the growth of the preexisting aerosol rather than forming a new growing mode of their own. The end times could also be linked to a weakening production of condensable vapors, as discussed in the following paragraph in the context of the DMD events.
On the majority of the NPF days, the particle growth phase is followed by a DMD event. Typically, the DMD phase starts in the afternoon around 15:00, approximately 6 h after NPF start (Fig. 7). The onsets of the DMD events are seemingly concurrent with the maxima of the wind speed, ABL height and the temperature. This could indicate that the DMD events are caused by particle evaporation, which is triggered by the increased saturation vapor pressure at elevated temperatures and the dilution of vapor concentrations due to ABL development and wind-induced mixing. This could also explain the observed summer maximum in the DMD event frequency, since these variables, which are likely to promote particle evaporation, obtain their largest values during summer. In addition, particle evaporation might be facilitated by the decreased photochemical production of condensable vapors after the maximum intensity of solar radiation.
Seasonal variation of the different phases of the NPF events observed in Hada Al Sham. The colored lines show the 20-point moving average for each of the different phases. The solid black lines show the times of sunrise and sunset, while the dashed black line shows the time of maximum solar radiation calculated based on the latitude of the measurement site.
A vast majority (
Figure 8 shows the seasonal variation of the particle formation rates
(
Both the formation and growth rates show a similar seasonal cycle, with
the largest values during the summer and early autumn (Fig. 8a, b). For the
growth rates, a summer maximum is often observed also globally, while the
formation rates peak in most locations during spring (Nieminen et al.,
2018). The different seasonal cycles of
In Fig. 8, the formation and growth rates are presented separately for the DMD and non-DMD NPF events. The comparison between these two cases is difficult due to the small number and uneven distribution of the non-DMD events. Regardless, during November–January, when the number of non-DMD events is especially large, the growth rates on non-DMD days are quite consistently lower than those on the DMD days (Fig. 8b). This would imply that the conditions between these cases are different already in the early stages of the NPF events, even though the DMD phase does not occur until hours later. The difference in the formation rates (Fig. 8a) is, however, less pronounced. One possible explanation for this could be higher concentrations of some semi/intermediate-volatility compounds on DMD days that would not participate in the initial particle formation, but would gain effectiveness with increasing particle size, due to decreasing Kelvin effect.
In Fig. 9, the formation rates are presented as a function of the relevant
meteorological variables (a–d), PM
Particle formation rate (
Figure 9b shows that the formation rate increases with increasing relative humidity (RH). This is the expected relationship between these two variables (Almeida et al.,
2013; Duplissy et al., 2016; Kürten et al., 2016), as water vapor is known
to participate in the cluster formation with sulfuric acid. However, in
ambient measurements such a correlation could be caused by processes that are
not necessarily related to the RH effect itself. For example, here the
higher RH could be related to the coastal origin of the air masses and
simultaneously to higher anthropogenic emissions from the coastal sector. To
examine this, the correlation was calculated separately for winds coming
from the S–W sector, where the
Higher formation rates seem to be favored by low wind speed and low ABL height (Fig. 9c and d). During low ABL conditions, the near-surface anthropogenic emissions are distributed into a smaller volume, which could then lead to higher vapor concentrations and particle formation rates. Analogously, the accumulation of emissions, per unit volume of air, from a spatially limited emission source area is increased during low wind speed conditions. Interestingly, the lowest event-time ABL heights are observed during the summer months (Fig. 9d), meaning that, in addition to the summer NPF events occurring earlier in the absolute sense (see Fig. 7), they also take place earlier with respect to the boundary layer development. This observation could be caused by the higher emissions during summer (see Fig. 8a), since if the concentrations of NPF precursors are higher, the onset of NPF events is likely to be more sensitive to an increase in solar radiation. We note that the RH dependence might also arise partly from higher RH in low ABL and wind speed conditions. However, since the correlation of RH and either ABL height or wind speed is weaker than that between RH and the formation rate, we expect this connection to be of secondary importance.
In the end of Sect. 3.1, we discussed briefly the possible interactions
between NPF and mineral dust, which is likely the major component of
PM
Out of the included variables, the strongest correlation is found between
particle formation rates and CS (Fig. 9f). This positive correlation is
quite interesting, since the concentrations of vapors participating in NPF
are expected to decrease with increasing CS due to their faster loss rate.
However, this is generally valid only if the sources of the CS and the
condensing vapors are independent from one another. Here, this is likely not
the case, but instead the increasing CS presumably represents increasing
contribution from the (primary) anthropogenic emissions and is therefore
simultaneously linked to higher concentrations of NPF precursor vapors. This
is supported by the observation that both the CS and
Figure A4 shows the particle growth rates as a function of the same
variables as the particle formation rates in Fig. 9. Overall, the
correlations are qualitatively similar but weaker in the case of GRs.
Similarly to the formation rates, the strongest correlation is found with
the CS (
The analysis of the aerosol number size distribution measurements showed that NPF events are a highly frequent phenomenon in Hada Al Sham, with the fraction of NPF days accounting for 73 % of all the classified days. The high NPF frequency is likely explained by the high production of NPF precursor vapors, especially sulfuric acid, in the transported emission plumes from the coastal cities and industrial areas during the typically prevailing cloud-free and high-solar-intensity conditions. The fraction of nonevent days was only 4 %, and these days were shown to be linked to strong easterly winds that block the development of the sea breeze, which typically brings the polluted air masses to Hada Al Sham.
Most of the NPF events in Hada Al Sham displayed an unusual progression, where the diameter of the particle mode related to the NPF event started to decrease after the growth phase. Similar DMD events have been observed in other measurement sites as well, but in Hada Al Sham the frequency of these events was found to be exceptionally high (76 % of all NPF days). The DMD events were more frequent during the summer, and the average onset time of the DMD events was during the afternoon, approximately 6 h after NPF start.
The median particle formation and growth rates associated with the NPF
events were 8.7 cm
Overall, the findings of this study highlight the importance of anthropogenic emissions and photochemistry for NPF. Due to the transportation of emissions from urban and industrial areas, NPF events were found to be very frequent in Hada Al Sham, located tens of kilometers away from the major sources. The frequency and strength of NPF observed here implies that NPF events might contribute significantly to the budget of both ultrafine and CCN particles, making their health and climate effects relevant topics for further studies in this region. The local conditions at Hada Al Sham, with high levels of regional anthropogenic emissions but presumably low concentrations of biogenic vapors, also allow us to research anthropogenic NPF events in detail. However, further experiments with a broader spectrum of instruments are required for determining the vapors responsible for new particle formation and growth, as well as the underlying reasons for the occurrence of the DMD events.
Data used in this study are available from the corresponding author upon request (simo.hakala@helsinki.fi).
Time series of particle number size distributions during February 2013, April 2014, July 2013 and December 2013 illustrating the high frequency and typical characteristics of NPF, as well as periods of nonevent and undefined days, e.g., during 23–27 December 2013. The shown time series were selected so that they would cover different seasons with continuous data availability.
The average
An example illustrating a case where the late starting time of an NPF event is related to a delayed shift in the wind direction in Hada Al Sham, 5 February 2013.
Particle growth rate (GR
MIK, HAJ, MAA, HL,
APH and TH coordinated the measurement
program and carried it out with KN, LD, ASA, IIS and FMA. ASA, IIS and FMA also provided
resources and data curation. VV provided the essential means for
calculating and analyzing the air mass trajectories. AMS
produced the figures related to the OMI
The authors declare that they have no conflict of interest.
We would like to thank the editor, Fangqun Yu, and the two anonymous referees for their insightful comments that helped us improve the manuscript.
This research was funded by the Deanship of Scientific Research, King Abdulaziz University (grant no. I-122-30); the Academy of Finland (grant nos. 307331 and 316114); and the European Commission (grant no. 742206).Open access funding provided by Helsinki University Library.
This paper was edited by Fangqun Yu and reviewed by two anonymous referees.